1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus is capable of forming a high-quality image with favorable reproducibility and operability at low costs. This is why the electrophotographic image forming apparatus has been widely used in a copier, a printer, a facsimile machine, a multifunction machine having two or more of these functions just stated, and the like machine. The electrophotographic image forming apparatus includes, for example, a photoreceptor, a charging section, an exposing section, a developing section, a transfer section, and a fixing section. The photoreceptor is a member on whose surface an electrostatic latent image corresponding to image information is formed. The charging section serves to charge a surface of the photoreceptor. The exposing section serves to irradiate the charged surface of the photoreceptor with signal light to thereby form an electrostatic latent image. The developing section serves to supply toner to the electrostatic latent image formed on the surface of the photoreceptor, to thereby form a toner image. The transfer section serves to transfer the toner image formed on the surface of the photoreceptor to a recording medium. The fixing section serves to fix the toner image on the recording medium to thereby form an image.
As such a fixing device, there has been generally known a fixing device in a thermal fixing system in which a toner image on recording paper is fused with heat and fixed on the recording paper by a roller pair system constituted by a fixing roller and a pressure roller, as shown in Japanese Unexamined Patent Publication JP-A 2007-304440.
The fixing roller is a roller member having an elastic layer formed on the surface of a metal-made hollow metal core, such as aluminum, and a halogen lamp is provided inside the metal core as a heat source. The temperature of the fixing roller is controlled by controlling on/off of the halogen lamp based on a signal output from a temperature sensor provided on the surface of the fixing roller by a temperature control device.
The pressure roller is a roller member having a heat-resistant elastic layer such as silicone rubber formed on a metal core as a coating layer. The pressure roller is brought into pressure-contact with the surface of the fixing roller, and due to elastic deformation of the elastic layer of the pressure roller, a pressure-contact region called a nip portion is formed between the fixing roller and the pressure roller.
In the configuration above, the fixing device sandwiches recording paper having an unfixed toner image formed thereon at the nip portion, rotates the fixing and pressure rollers to transport the recoding paper, and fuses a toner image on a recording material with heat on the surface of the fixing roller to fix to the recording paper.
In such a fixing device, offset phenomena such as cold offset and hot offset are known to occur when the surface temperature of the fixing roller falls outside of a suitable temperature range. The cold offset is a phenomenon in which a part of toner that did not melt sufficiently due to shortage of heat transferred to the recording paper adheres to the fixing roller. On the other hand, the hot offset is a phenomenon in which a part of toner on the recording paper adheres to the fixing roller due to a weakened toner cohesive force caused by overheating of the toner on the recording paper.
In order to prevent image contamination on recording paper due to offset phenomena, a cleaning section composed of a cleaning web and the like that is brought into contact with a fixing roller is generally provided as shown in the JP-A 2007-304440. Although most of the toner adhering to the fixing roller is removed by the cleaning section, not all of the toner is removed.
Accordingly, in order to prevent offset phenomena, it is very important in the fixing device to control the temperature so that the surface temperature of the fixing roller falls within a suitable temperature range while the recording paper passes.
A suitable temperature range for the surface of the fixing roller varies depending on recording paper transport speed of an image forming apparatus provided with the fixing device. Specifically, the suitable temperature range tends to shift to the high-temperature side with increase in the recording paper transport speed and to the low-temperature side with decrease in the recording paper transport speed. This is because the contact time of the recording paper and the surface of the fixing roller is short when the recording paper transport speed is fast, and the surface temperature of the fixing roller is required to be relatively high to transfer sufficient heat from the surface of the fixing roller to the recording paper. In addition, the contact time of the recording paper and the surface of the fixing roller is long when the recording paper transport speed is slow, where excessive heat is transferred from the surface of the fixing roller to the recording paper unless the surface temperature of the fixing roller is restrained.
A so-called four-cycle image forming apparatus is generally designed so that the recording sheet transport speed is almost the same for the case of forming color images and the case of forming monochromatic images on recording paper. However, the distance between transported recording sheets of recording paper is different between the case of color images and the case of monochromatic images, and the number of processed sheets per unit of time is greater in the case of monochromatic images. Note that, a four-cycle image forming apparatus forms toner images of four colors cyan (C), magenta (M), yellow (Y), and black (B) by a pair of visualized image forming units, which are overlaid to form a color image.
Accordingly, in a fixing device provided in the four-cycle image forming apparatus, there are two non-offset ranges of a suitable temperature range (non-offset range) for fixing color images and a suitable temperature range (non-offset range) for fixing monochromatic images, and the common non-offset range is sufficiently wide. It is therefore easy to set a control value (target value) for the surface temperature of the fixing roller within the common non-offset range and to control the surface temperature of the fixing roller within the suitable temperature range in both the case of fixing color images and the case of fixing monochromatic images. As a result, the four-cycle image forming apparatus is capable of avoiding the offset phenomena easily.
Meanwhile, in a recent image forming apparatus employing a four-drum tandem engine, that is, in an image forming apparatus in which four sets of visualized image forming units respectively corresponding to four colors of C, M, Y, and B are arranged in series, there is an increasing demand for designing so as to provide faster recording paper transport speed in the case of forming monochromatic images than the case of forming color images. According to this demand, the recording paper transport speed is largely different between a monochromatic mode forming monochromatic images and a color mode forming color images. Even with such design, it is possible to increase the number of processed sheets for monochromatic images per unit time without deteriorating image quality in formed monochromatic images.
As described above, in the image forming apparatus that is designed to increase the number of processed sheets for monochromatic images, the common non-offset range in the non-offset range for fixing color images and the non-offset range for monochromatic images is significantly narrow. When the common non-offset range is significantly narrow in this manner, it is difficult to control the surface temperature of the fixing roller within the common non-offset range even when a control value (target value) for the surface temperature of the fixing roller is set within the common non-offset range, resulting that offset phenomena are easily caused.
Further, for the paper passing operation from a standby state, the temperature needs to be set to enable paper passing both in the monochromatic mode and the color mode. Thus, during the standby state, the surface temperature of the fixing roller needs to be controlled between a lower limit of fixing temperature for the monochromatic mode and an upper limit of fixing temperature for the color mode, and be quickly switched between the monochromatic mode and the color mode in the subsequent paper passing operation.
For accurately controlling the surface temperature of the fixing roller in a fixing device for color, an external heating section such as an external heating roller or an external heating belt that is in contact with the surface of the fixing roller is more suitable than an internal heating section such as a halogen lamp provided inside the fixing roller. Accordingly, it is important to control the temperature by the external heating section for controlling the surface temperature of the fixing roller to fall within a narrow temperature range.
For example, in one method for temperature control by the external heating section, contact/separation of the external heating section is controlled based on information of the surface temperature of the fixing roller so as to instantly raise or lower the surface temperature of the fixing roller. It is possible to control the surface of the fixing roller at a predetermined temperature by bringing the external heating section into contact with the fixing roller to increase the surface temperature of the fixing roller or separating the external heating section from the fixing roller to prevent overheating of the fixing roller according to a print mode.
However, there is also a problem in the contact/separation operation for the external heating section. The fixing roller having the external heating section is generally provided with the cleaning section as described above on the upstream side of the external heating section in a rotational direction of the fixing roller. Even when offset phenomena are caused, the cleaning section reduces much of the toner contamination on the fixing roller. However, since the cleaning capability of the cleaning section is not perfect, the toner contamination accumulates on the external heating section that is in contact with the fixing roller. As the toner accumulates on the external heating member to a certain extent, the toner adhering to the external heating section is sometimes peeled from the external heating section collectively and transferred to the fixing roller by the vibration caused when the external heating section is brought into contact with the fixing roller. When a certain quantity of toner is transferred from the external heating section to the surface of the fixing roller, the toner adheres to the recording paper during paper passing by the fixing roller, thus causing a problem that the print face is contaminated.
In order to solve the problem, as shown in the JP-A 2007-304440, a fixing device has been proposed that is provided with a control section that controls the contact/separation section so as to bring the external heating section into contact with the fixing roller at such a timing that an initial contact portion, which is a portion of the external heating section on the fixing roller where the external heating section is initially brought into contact with the fixing roller, does not make contact with the recording paper in one rotation of the fixing member.
However, in the invention of the JP-A 2007-304440, since the external heating section is not provided with a cleaning section, the control of the contact/separation operation of the external heating section for the fixing section is more complicated in order to further prevent adhesion of toner and the like to the external heating section. Further, even when the adhesion of the toner and the like to the external heating section can be prevented completely, a cleaning web for cleaning the fixing roller is essential and the apparatus becomes large by arranging the cleaning web.